I was recently asked this question, in just that way by a less than happy client. The short answer is the Freedom of Information Act. The slightly longer answer is that anything tax deductible (home mortgage, retirement plan contribution, charitable contributions, etc.) falls under the mantle of public information and can be found by those who are looking.
Who's looking? There are firms whose sole function is to mine data of this nature and then sell it to interested parties. Those interested parties then use the data to promote their cause (usually a sale of some sort). The interested parties add their own interpretation of the information and turn it into something just this side of mis-information and scare tactics. For example, many of our clients received letters advising them that their retirement plan is "out of compliance" when nothing could be further from the truth. The sender of this letter purchased the mined data and then set out to contact the plan sponsors with this less than truthful information.
It comes down to the same old saw. In this day and age of widely available information, vigilance is our best tool. Others may be able to obtain your information, but don't let them use it against you. If you would like to discuss this further as it pertains to your personal plan or situation, please call our office at your convenience.